"Product liability insurance, overhead, leases, inventory tax, use tax, state/federal witholdings" and:
General insurance (for when you wreck that H&H presentation grade double rifle or the shop burns down with all your tools and a quarter million dollars worth of customer guns); OSHA compliance; security, including alarms; local zoning law compliance; local and state business licenses; the list can go on forever. Capital costs are high. Just a set of headspace gauges will run $90, a chambering reamer about the same, and that is for each caliber. You can run through $50k pretty quickly just setting up a shop. Sure you can farm out work, or rent reamers, but every customer who has to wait too long is a customer lost.